Discharge lamps, specifically HID (high-intensity discharge) lamps are used for a large area of applications where high luminous flux is required. Especially in the automotive field, HID lamps are used as vehicle headlamps.
A discharge lamp comprises a sealed discharge vessel, which may be made e.g. from quartz glass, with an inner discharge space. Two electrodes project into the discharge space, arranged at a distance from each other, to ignite an arc there between. The discharge space has a filling comprising a rare gas and further ingredients such as metal halides.
An important aspect today is energy efficiency. The efficiency of a discharge lamp may be measured as lumen output in relation to the electrical power used. In discharge lamps used today for automotive front lighting an efficiency of about 90 lumen per Watt (lm/W) is achieved at a steady state operating power of 35 Watt.
WO 2008/110967 describes a HID lamp for a vehicle headlight with reduced nominal power of 20-30 W. In an embodiment, a 25 W lamp has a discharge vessel of ellipsoid shape with electrodes provided at an optical distance of 4.2 mm. An inner diameter of the discharge vessel is 2.2 mm and an outer diameter is 5.5 mm. A filling is provided within the discharge vessel of Xenon at a cold pressure of 17 bar and metal halides 36 wt-% NaI, 24 wt-% ScI3 and 40 wt % ZnI2. An outer bulb is provided around the discharge vessel at a distance d2 which is sealed a filled with a filling gas of reduced heat conductivity λ measured at 800° C. A heat transition coefficient calculated as thermal conductivity λ of the outer bulb filling gas divided by the distance d2 between the discharge vessel and the outer bulb is below 150 W/(m2K).